hls, rtmp: print clearer error messages (#501)

in case a stream doesn't contain a H264 track
or a AAC track.

internal/core/hls_remuxer.go

@@ -287,7 +287,7 @@ func (r *hlsRemuxer) runRemuxer(remuxerCtx context.Context, remuxerReady chan st
 	}
 
 	if videoTrack == nil && audioTrack == nil {
-		return fmt.Errorf("unable to find a video or audio track")
+		return fmt.Errorf("the stream doesn't contain an H264 track or an AAC track")
 	}
 
 	var err error

internal/core/rtmp_conn.go

@@ -264,7 +264,7 @@ func (c *rtmpConn) runRead(ctx context.Context) error {
 	}
 
 	if videoTrack == nil && audioTrack == nil {
-		return fmt.Errorf("unable to find a video or audio track")
+		return fmt.Errorf("the stream doesn't contain an H264 track or an AAC track")
 	}
 
 	c.conn.NetConn().SetWriteDeadline(time.Now().Add(c.writeTimeout))